I had good luck using the MAS Cell O Fill to bond a bare wood combing to my fiberglassed kayak. I put the Cell O Fill on the bottom of the bare wood combing and on the already fiberglassed and sanded cockpit. I clamped the combing in place just as if I were glueing it. After it dried, I sanded it and then I then fibreglassed it, using 4 oz left-over fiberglass I bias cut so that it would wrap around the combing. The Cell O Fill did a nice job of filling in gaps and looks nice.

The Cell O Fill came with my kit.  I mixed the MAS resin for two minutes and then added the Cell O Fill until it thickened...it seemed like a good idea at the time...but I haven't been able to find any directions. I mixed it to the consistency of mayonaise. It's cloudy when you mix it but it dries clear.

I actually posted to this forum for info on Cell O Fill last week but didn't get any answers, so I did a little Googling and did this pretty much on a wing and a prayer. So far so good. I figure that, if anything, the fiberglass will provide additional strength to the Cell O Fill.

In my Googling, I came on a West Marine video on West fiberglassing products and found out they have several similar additives for resin that are used for structural bonding, so I think using products like Cell O Fill for bonding is standard practice. I think it's best to use fillers like Cell O Fill rather than resin alone to bond bare wood to fiberglass. And I think it's a very good idea to fiberglass the whole thing afterwards for strength and water resistence. 

Hi Dr. Pepper,

Coating the coaming and sanding it before installing it should work fine. If you can keep the epoxy away from the faying surface of the coaming (cover it with masking tape while pre-coating, for example) you will still have a bare wood to wood joint. That would give you a "chemical" bond.

If you have cured epoxy on one or both surfaces, you would need to make sure to scrub off any traces of amine blush, and then sand the cured epoxy to a dull matte surface before bonding. You would then have a "mechanical" bond, which is often said to be not quite as strong as a chemical bond.

But I have asked a few prominent small boat designers if they have ever heard of a properly executed epoxy mechanical bond failing, and they have not. Chemical bonds are nice when you can get them, but sometimes our work schedules just don't make that possible. So it's reassuring to hear that mechanical bonds are good enough.

Old Yeller

Thanks for the input. Since I already applied epoxy to the deck, I assume a fully "chemical" bond is out of the question already. (I assume "chemical" really means that the epoxy has a chance to penetrate the wood while curing.) Anyway, I will make sure to carefully sand the deck where the coaming will be attached. Given that the coaming isn't really structural, I am hoping that a "mechanical" bond will be strrong enough.

I will use the Cell-O-Fill for attaching the coaming to the deck, thus hopefully further strengthening the bond.

Thanks!

"I will use the Cell-O-Fill for attaching the coaming to the deck, thus hopefully further strengthening the bond."

Definitely. A thickening agent is required for any joint made with epoxy. Otherwise there is a danger that too much of the epoxy will drain out of the joint. For the same reason, epoxy joints don't like massive clamping pressures. Just enough to close closable gaps and hold the pieces in position.

Old Yeller